Heretofore, in grinding a workpiece with a grinding wheel, grinding burn, thermal stress and the like of the workpiece caused by the grinding heat are prevented by supplying coolant toward a grinding point between the workpiece and the grinding wheel for cooling and lubrication. However, where a superfluous volume of coolant is supplied toward the grinding point between the workpiece and the grinding wheel, a dynamic pressure is generated in the coolant between the workpiece and the grinding wheel. In particular, where the workpiece has a hole or groove, the same causes the dynamic pressure to fluctuate, which gives rises to a problem that the machining accuracy of the workpiece is deteriorated due to a relative displacement between the workpiece and grinding wheel. Patent Document 1 discloses a technology for preventing the machining accuracy from being deteriorated due to such a dynamic pressure generated in the coolant. In the technology described in the Patent Document 1, there is provided a coolant supply device capable of switching into two high and low steps the pressure of coolant supplied to a coolant nozzle which supplies coolant toward a grinding point at which the grinding wheel contacts a workpiece. The coolant pressure is switched into a high pressure during a rough grinding wherein the feed rate of the grinding wheel toward the workpiece is high, but into a low pressure during a finish grinding wherein the feed rate is low, as well as during a spark-out grinding. Thus, the machining accuracy is prevented from being deteriorated due to the dynamic pressure generated in coolant.    Patent Document 1: Japanese Utility Model Application No. 57-157458 (pages 1-3 and FIG. 2)